Page printers intrinsically capture an entire page image before any images are placed on paper. This enables substantial flexibility in the software that controls the printer, since it isn't necessary to compose a page from top to bottom. In printers which employ laser printers as the "print mechanism", data must be provided at a speed that is fast enough to keep up with paper that moves past the imaging drum. In such printers, formatting is either performed on the host computer, with large volumes of rasterized data shipped to the printer at high speed, or on a formatter within the printer. Since a laser printer mechanism operates at a constant speed, if rasterized data is not available to the print mechanism when a previous segment of data has been imprinted on the paper, a "print overrun" occurs and the page is not printable on the specific model printer. Further details on rasterization of data can be found in "Fundamentals of Interactive Computer Graphics" by Foley et al., published by Addison-Wesley Publishing Company, 1982.
To prevent print overruns, several algorithms are in common use. In the first, a full raster bit map of an entire page is stored so that the print mechanism always has rasterized data awaiting its action. Unfortunately, at 300 dots per inch resolution this technique requires approximately a megabyte of memory for each page. Because laser printers achieve their rated speeds by pipelining, more than one megabyte of memory is needed to run the printer at its rated speed. Otherwise, composition of a following page cannot begin until a prior page has been ejected to the output tray. The provision of such large amounts of memory adds significantly to the cost of a printer and is avoided, if possible.
A second algorithm constructs a display list and enables considerably less memory to be utilized in the print rasterization process. A page description is built in two steps: during formatting, a page description received from a host is converted into a series of simple commands, called display commands, that describe what must be printed. Once conversion of the page is complete, the second step prepares the display command list for printing. This step entails parsing through the display commands and rendering the described objects into a raster bitmap. At worst, only one full page raster bitmap is needed--even when pipelining--because the same memory is used for each page.
In order to further reduce the amount of memory, the display command list algorithm has seen a further development. Display commands are sorted according to their vertical position on the page. Once the sort has occurred, the page is divided into sections called page strips, and each page strip is passed, sequentially, to the print mechanism for printing. When display commands within a page strip are rendered into rasterized data at a fast enough pace, the same memory used to store a first page strip can be reused for a subsequent page strip further down the page (once the first page strip has been transferred to paper).
The Laser Jet III laser printer, manufactured by the assignee of this application, employs a display command list algorithm and utilizes three raster buffers. The first buffer is reused on the fourth strip of a page; the second buffer reused on the fifth strip, etc. For an eight page per minute printer, little time is left between the finishing of strip one and the time when strip four will be required by the print mechanism. If the strip is not delivered in time, a "print overrun" occurs. While the size of a raster buffer can be substantially increased to avoid a print overrun, this defeats the purpose of the display list algorithm, to wit, a reduction in the amount of memory required for the print action.
So long as a laser printer deals with normal text and raster graphics, a print overrun will ordinarily not occur. However, if vector graphics are employed, even a simple-looking page can outrun the capability of a processor to rasterize a display list rapidly enough to keep up with the printer mechanism.
It is therefore an object of this invention to provide an improved method and system for substantially reducing the possibility of print overruns in printers.
It is another object of this invention to provide an improved method for preventing print overruns which does not require added memory.
It is yet another object of this invention to provide an improved system for preventing print overruns that enables a printer to operate at a high page per minute print rate.